Electromagnetic Interference Shields for Electronic Devices

ABSTRACT

An EMI shield for personal computers, cellular telephones, and other electronic devices is constructed from thermoformable polymeric material which is then metallized on all surfaces by vacuum metallization techniques to provide an inexpensive, lightweight, yet effective EMI shield.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/659,838,filed Sep. 10, 2003, which is a continuation of application Ser. No.09/732,235, filed Dec. 7, 2000, now U.S. Pat. No. 6,624,353, which is acontinuation of application Ser. No. 08/958,595, filed Oct. 29, 1997,now U.S. Pat. No. 6,570,085, which is a division of application Ser. No.08/463,297, filed Jun. 5, 1995, now U.S. Pat. No. 5,811,050, which is acontinuation-in-part of application Ser. No. 08/254,250, filed Jun. 6,1994, now abandoned, the complete disclosures of which are incorporatedherein by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

This invention pertains to shielding apparatus for containing highfrequency electromagnetic radiation within a personal computer, cellulartelephone, or other electronic instrument.

Electromagnetic compatibility (EMC) is a broad term used to describeelectromagnetic interference (EMI), radio frequency interference (RFI)and electrostatic discharge (ESD), and the above terms are often usedinterchangeably.

The fact that electronic devices are both sources and receptors of EMIcreates a two-fold problem. Since electromagnetic radiation penetratingthe device may cause electronic failure, manufacturers need to protectthe operational integrity of their products. Secondly, manufacturersmust comply with the regulations aimed at reducing electromagneticradiation emitted into the atmosphere. Proper design is necessary toprevent the device's function from being disrupted by emissions fromexternal sources and to minimize its system's emissions.

Today, plastics are replacing metals as the material for electronicenclosures since plastics offer increased design flexibility andproductivity with decreased cost. The switch from metal to plastics as ahousing material for electronic equipment has contributed to concernover EMI shielding. Plastics are insulators, so EMI waves pass freelythrough unshielded plastic without substantial impedance or resistance.Additionally, ever increasing device miniaturization and the increase inclock speeds of microprocessors used in computing devices makes it moredifficult to handle the EMI pollution these faster computers generate.So a variety of technologies using metal/polymer combinations andcomposites are being used as a shielding material in electronic devices.

Current methods for shielding of electromagnetic interference (EMI)include the use of metal housings, metal filled polymer housings, metalliners for housings, and conductive coatings for the interior of rigidpolymer or composite housings.

Metal coatings for rigid plastic housings are applied through use ofconductive paints or through application of metal platings applied bychemical plating (electroless plating), by electroplating, or throughvacuum metallization. In addition, metal foils with adhesive backingsmay be applied to the inside of plastic cases for electronic instrumentsto achieve shielding requirements. Zinc Arc spray techniques are alsoavailable to apply a metal coating to a plastic housing.

Another shielding material is provided through the use of metal fiberssintered onto a polymeric substrate as is taught in U.S. Pat. No.5,226,210, and commercially produced as #M 610D ThermoformableEMI-shielding material by the Minnesota Mining and Manufacturing Companyof St. Paul, Minn.

Each of the current shielding methods has shortcomings. The majordisadvantages of plating are its high cost, complex process cycles, andits application is limited to only certain polymer resins. Metal-filledresins for injection molding suffer from poor conductivity compared tometals. The conductive polymer resin is very expensive and complex shapemolding is difficult from flow and uniformity perspectives.

Three general types of conductive metal-bearing paints are in generaluse. Silver paints have the best electrical properties but, they areextremely expensive. Nickel paints are used for relatively lowattenuation applications and are limited by high resistance and poorstability. Passivated copper paints have moderate cost and lowerresistivity, but also lack stability. All paint applications havedifficulties with coating uniformly, blow back in tight areas andrecesses depending on part complexity, and application problems whichcan lead to flaking. Paints also fail ESD testing over 10 KVA.

EMI shielding through the use of metal cases for the personal computeror other electronic device may not always be desired because of concernsabout weight and aesthetics, with weight being a serious concern forlaptop computers or portable and handheld devices of any types. The useof a metal shroud to line a plastic case improves over the metal case inaesthetic and design concerns for the outside of the housing but resultsin an increased assembly step and little weight minimization. Metal alsolacks the ability to be formed into complex shapes often taking upunnecessary room adjacent to the circuitry and assembled electricalcomponents.

The use of coated plastic housings for electronic devices, includingmicrocomputer and cellular telephones, may not provide a suitablesolution when one considers that personal computers currently offeredmay operate at clock speeds of 100 MHz which gives rise to opportunitiesfor EMI generation not previously confronted in the personal computerindustry. Further, the ever increasing clock speeds of the personalcomputers being offered makes effective shielding more and morechallenging since any breach in the shield which has one dimension inexcess of 0.23 inch may allow substantial EMI leakage, causing the unitto fail United States Federal Communication Commission standards.

The use of metallic coatings on plastic housings presents certainmanufacturing and service concerns. A slipped tool used during assemblyor a repair can cause a scratch in the metal coating of sufficient sizeto cause a slot antenna, thereby making the case totally useless, andthereby leading to a costly item being discarded with little feasibilityfor successful recycling.

The seams of a metal plated plastic housing will act like slot antennaeunless the housing sections are conductively joined by the use ofoverlapping joints, conductive gaskets, or conductive tape. When thehousing must be opened for a repair or retrofit, it can be understoodthat some of the conductive interconnection may be degraded by theactivity of disassembly.

Further background on prior art methods and characteristics of shieldingmethods may be examined in “EMI/RFI Shielding Guide,” published by theGE Plastics Division of the General Electric Company, and in “theDesigner's Guide to Electromagnetic Compatibility” by Gerke & Kimmel,Supplement to EDN Magazine, Volume 39, No. 2, (Jan. 20, 1994).

BRIEF SUMMARY OF THE INVENTION

This invention pertains to EMI shielding for personal computers,cellular telephones and other electronic devices which are subject toPart 15 of the FCC Rules. A thermoformable polymeric sheet is formedinto an enclosure sized and shaped to enclose an EMI emitting subsystemor component. The thermoformed polymeric enclosure is then metallized onall or selected surfaces by vacuum metallizing techniques where thethermoformed enclosure is placed in a vacuum chamber, treated with anionized gas, and then metallized by the use of aluminum or other metalbeing vaporized by use, for example, of a current-passing tungstenfilament, or other vaporization techniques. The enclosure is rotatedwithin the chamber to allow metallization of all desired surfaces.Masking may be employed when certain regions or surfaces are preferrednot to be metallized. The enclosure is thereby provided with wallshaving a polymeric substrate provided on desired surfaces with a vacuummetallized layer. The vacuum metallized layers are of sufficientthickness to make the surfaces of the enclosure electrically conductive.The enclosure is formed in the shape and size necessary to house andshield the EMI emitter; for example in the case of a personal computer,the enclosure may serve as a thin-walled case within the rigid outerhousing of the computer. Alternatively, the enclosure may be formed tofit as an insert within a device's housing as a substitute for a metalinsert shield, or the enclosure may be shaped and sized to contain onlycertain components which are emitters of, or susceptible to, EMI. Gangsof metallized enclosures may be devised with electrical isolation asdesired provided by gaps in the metallization layers. Differentelectronic devices will require varying degrees of attenuation orshielding effectiveness. The enclosure may be coated on all surfaces orselectively coated for certain applications.

Thermoformed shapes have previously been vacuum metallized withthin-film coatings (350 to 1000 angstroms or 0.035 to 0.10 microns) butonly for their reflective metallic appearance. Conventional thin-filmvacuum metallizing is not adequate to dissipate EMI. Existing equipmentfor metallizing thermoformed shapes for ornamental reflective appearancepurposes is not suitable for application of relatively thick thin filmas is required to provide suitable surface impedance to allow effectiveEMI dissipation.

Many polymeric materials are thermoformable. Formability, thickness,melt strength, shrinkage, flame retardency, and other properties arefactors determined by the end user of the finished product. Extrudedroll or sheet materials suitable for thermo forming includeAcrylonitrile-Butenate-Styrene (ABS), polystyrenes, cellulose polymers,vinyl chloride polymers, polyamides, polycarbonates, polysulfones, andolefin polymers such as polyethylene, polypropylene, polyethyleneterephthalate glycol (PTG) and methyl methacrylate-acrylonitrile(co-polymers).

Use of these polymers with additional fillers such as carbon black,graphite, and metal fibers, add to the shielding effectiveness forabsorbing more of the lower electro-magnetic wavelengths.

The polymeric enclosures are not metallically coated until after thethermoforming process. Because the forming process stretches or drawsthe material into corners and recesses, it would also draw or thin themetallic coating making its uniformity vary in different areas on theformed shape if coatings were applied prior to forming.

After forming, metallic coatings may be applied to the shapes by avariety of vacuum deposition techniques such as thermal evaporation,cathode sputtering, ion plating, electron beam, cathodic-arc, or vacuumthermal spray.

Because a thermoformed enclosure is used, the shield is of reducedweight and if damage occurs to the thermoformed shield duringmanufacturing or repair of the electronic device, a less costlyreplacement item is needed.

The use of interlockable enclosure bodies which may snap together orotherwise be mechanically held in assembled state, permits the walls ofthe shield to be in conductive contact and reduces or eliminates theneed for conductive tape or conductive gaskets while providing aneffective EMI shield. Further securing means may be employed, such as byuse of conductive adhesive, laser welding, or heat sealing.

It is accordingly an object of the invention to provide an EMI shieldwhich may be thermoformed into a desired shape with metallizationapplied on all surfaces of the shield.

It is another object of the invention to provide an EMI shield whichprovides an easy-to-manufacture shield with excellent attenuation of thestrength of electric or magnetic fields.

Another object of the invention is to provide an inexpensive EMI shieldthat will not be totally degraded by a scratch on one surface of theshield.

Another object of the invention is to provide an EMI shield which islight weight.

Another object of the invention is to provide an EMI shield which may benested for shipment.

Another object of the invention is to provide an EMI shield withsuperior conductive wall coupling structure.

Another object of the invention is to provide an EMI shield which willnot need application of conductive tape or gaskets to provide adequateshielding.

Another object of the invention is to provide an EMI shield whichincreases resistance of the shielded components to corrosive atmosphericconditions.

These and other objects of the invention will become understood from areview of the detailed description of the invention which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a laptop personal computerhaving the shield invention installed therein.

FIG. 2 is an enlarged view of a typical cross section of a sidewall ofthe preferred embodiment shield invention.

FIG. 3 is a schematic view of the typical apparatus used for applyingmetal deposition to the polymer thermoforms of the preferred embodiment.

FIG. 4 is a plan view of the preferred embodiment of the invention inits unfolded arrangement.

FIG. 5 is an enlarged perspective view of the interconnecting edges ofthe preferred embodiment shield invention.

FIG. 6 is an enlarged cross section of the engagement between a covermember and a base member of an alternate embodiment of the shieldinvention.

FIG. 7 is an enlarged cross section of a second alternate embodiment ofthe shield invention showing the cover thereof in phantom in an openposition.

FIG. 8 is a graph of relative coating thickness as a function of vaporstream incident angle; and

FIG. 9 is a comparison of work range injection molded vs. thermoformedparts distance from part to vapor source.

FIG. 10 is a graph showing shielding effectiveness as a function offrequency for silver acrylic paint and silver vacuum deposited coatings.

FIG. 11 is a graph showing shielding effectiveness as a function offrequency for aluminum vacuum coating on one side and aluminum vacuumcoating on two sides.

FIG. 12 is a graph showing shielding effectiveness as a function offrequency for aluminum vacuum coating on one side and aluminum vacuumcoating on two sides, with two coats of aluminum.

FIG. 13 is a graph showing shielding effectiveness as a function offrequency for a silver vacuum metallized coating, an aluminum vacuummetallized coating, and a copper vacuum metallized coating.

FIG. 14 is a graph showing magnetic field shielding effectiveness as afunction of frequency for aluminum vacuum coating on one side andaluminum vacuum coating on two sides.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawing figures and in particular to FIG. 1, theinvention 2 is shown in place as a component of a laptop personalcomputer 4. Bottom case 6 of computer 4 is provided with power supplymodule 7 stationed therein. Invention 2 encloses the mother board of thecomputer 2, including the central processing unit, memory storage chips,input-output circuit components and the like (not shown). Top case 8overlies invention 2 when invention 2 is placed within bottom case 6 ofcomputer 4. Top case 8 includes keyboard 10 and visual display 12 whichare interconnected to associated circuitry housed in invention 2 byleads 14. Power supply 7 and input/output ports 9 are electricallyconnected to associated circuitry housed in invention 2 by cables 16.

FIG. 2 discloses a cross section of a wall of invention 2, showing apolymeric substrate 25 having conductive metallization layers 27 and 29applied thereto by vacuum metal deposition techniques. Each of layers 27and 29 are a relatively thick, thin film of metal, preferably ofaluminum, copper, or silver. In the preferred embodiment, aluminum isused, and is applied to the polymeric substrate 25 after the polymericsubstrate has been thermoformed into a desired enclosure shape and thenits surface is modified by bombardment by an ionized gas in an evacuatedchamber or by other means suitable to increase surface tension of thesubstrate 25. The substrate 25 is then placed in an evacuated chamberwhere a metal is vaporized and deposited on the substrate 25 on thesurfaces thereof, through rotation of the substrate 25 about itself andabout the metal vapor source. Substrate 25 has been earlier formed intoa desired shield shape before application of the metallization layers 27and 29 in order to achieve a uniform thickness of metallization over thesurfaces of substrate 25. By thermoforming substrate 25 beforesubjecting it to the metallization step, problems with thinning of themetallization layers 27 and 29 at corners, bends and the like, whichmight occur if the substrate were formed after metallization is applied,are avoided. If desired, certain regions of substrate 25 may be maskedto prevent deposit of metal film on those regions.

By applying a relative thick film (between 1.0 and 50 microns thick),which has a surface impedance of less than one ohm per square per inch,a suitably conductive layer of metallization is achieved which providesa low surface impedance and hence effective EMI attenuation. Theapplication of metallization layers 27 and 29 to opposing sides ofsubstrate 25 increases the EMI attenuation achieved.

FIG. 4 discloses the preferred embodiment of the invention 2 in itsunfolded state. Polymeric sheet material is thermoformed into a desiredshield blank 30 by conventional methods. Blank 30 comprises firstsection 18 and second section 20 interconnected by hinge region 22, allof which are formed from continuous polymeric sheet of generally uniformthickness. By use of thermoformable material, it can be understood thatlight weight is realized and that unassembled shield blanks 30 may benested for shipment.

From FIGS. 4 and 5, it can be seen that the edges 24 of first section 18of blank 30 are formed to fit in complementary engagement with the edges26 of second section 20 of blank 30. In particular, edges 24 of firstsection 18 are provided with shoulder recesses 19 wherein ridges 21 ofsecond section 20 are receivable, such that the periphery 34 of secondsection 20 is overlapped by the periphery 32 of first section 18 whenfirst section 18 and second section 20 are folded about hinge region 22into engagement. By this overlap, EMI shielding is made substantiallythorough as the touching engagement of overlapping second section 20 onfirst section 18 provides electrical conductivity between the sections.Conductive adhesive or conductive tape may be added to the seam formedbetween first section 18 and second section 20 to ensure sufficient EMIshielding in the seam region. Hinge region 22 provides a conductive pathbetween first section 18 and second section 20. To aid in reducing gapsin EMI shielding effect, conductive adhesive 36 may be applied toflanges 38 and 40 of blank 30, which will come into abutment whenenclosure bodies 18 and 20 are folded about hinge 22 for edgewiseengagement.

FIG. 5 illustrates further the novel mechanical locking means of thepreferred embodiment EMI shield 2. Boss 39 is formed upon flange 40 offirst section 18 and is engageable with recess 41 formed in flange 38 toprovide additional retention forces when first section 18 and secondsection 20 are engaged.

FIG. 3 discloses apparatus for vacuum deposition of metallic coating onthermoformed blank 30 which is placed on carrier 62 in evacuable chamber60. Chamber 60 is evacuated and a gas, including a gas from the groupincluding Argon, Nitrogen, Oxygen, CF₆ and SF₄, is passed into chamber60, excited by an electric charge, and the resulting ionized gas servesto modify the surfaces of blank 30. Chamber 60 is again evacuated andcarrier 62 is caused to revolve around tungsten filament 64 which isenergized electrically to provide energy to vaporize metal 66, themolecules of which travel from filament 64 and are deposited on blank30. Blanks 30 may alternatively be retained to planetary mount 68 whichrotates about itself as it revolves about filament 64 in the directionof arrows a. Control 68 is associated with chamber 60 to causeevacuation of the chamber, introduction of gas for surface modificationof the blank, and energization of the filament.

FIG. 6 illustrates another embodiment of the shield invention wherein athermoformed case 130 is formed of formable sheet polymer. Lid 132 islikewise thermoformed of sheet polymer into a complementary shape. Lid132 is provided with spring element 134 about its periphery, springelement 134 being formed upon lid 132 and being urged into touchingengagement with inner surface 138 of case 130. After case 130 and lid132 are suitably thermoformed, they are passed through a vapor metaldeposition operation where a metal film is deposited on selectedsurfaces, including all surfaces thereof if desired. In the embodimentof FIG. 6, metal film of thickness between 1.0 and 50.0 microns isdeposited upon outer surface 140 of lid 132 and a similar metal layer isvapor deposited upon outer surface 142 of case 130 and upon region 138of case 130. Inner floor surface 144 of case 130 is polymeric, havingbeen masked to prevent deposit of metallization thereon. In thisembodiment, conductivity of surface 144 has been avoided in order toprevent interference with surface circuitry of a component carryingboard which may be installed within the shield of FIG. 6.

FIG. 7 discloses another alternative embodiment of the shield inventionwherein shield body 102 comprises a thermoformed polymer base 104 with ahinged cover member 106. Shield body 102 is provided with projections108 upon the upper area of first sidewall 110. Cover 106 is fixed byhinge 122 to case 104 and is provided with indents 112 which are formedin the outer leg 114 of U-shaped recess 116. Recess 116 extends aroundthe periphery of cover 106 and is provided to permit touching engagementof outer leg 114 with the sidewalls of base 104. At region 124, forexample, conductive surface contact is provided between base 104 andcover 106.

Shield 102 is first thermoformed into the desired shape from sheetpolymer material and then a metallic layer is vapor deposited on all orselected surfaces of shield 102. The metallic layer is suitably thick toprovide excellent surface conductivity thereby providing excellent EMIattenuation.

It can be further understood that thermoformed shapes such as enclosurebodies 18 and 20 of FIG. 4 may be ganged together by interconnectedwebs, such as hinge 22 of FIG. 4, wherein a metal deposition layer isapplied to the outer surfaces of shapes 18 and 20 respectively while nometallization is applied to hinge 22, such result being effected byapplication of masking to hinge 22 before it is passed into theevacuated chamber where metal is to be vapor deposited thereon. Theresulting gang of metallized shapes may then be used to provide EMIshielding to discrete through adjacent components or component groups ona circuit board where the components or groups are separated by adistance substantially equivalent to the dimension of the web (hinge 22)between sections 18 and 20. The absence of conductive metal coating onhinge 22 prevents conduction of electrical signals from one shape to theother while providing an efficient system of creating inexpensive EMIshield gangs.

Vacuum deposition is the vaporization and subsequent condensation of anymetal or compound onto a substrate in a substantial vacuum. Althoughcommonly referred to as a single process, the deposition of thin filmsby vacuum evaporation consists of several distinguishable steps, namely:transition of a condensed phase, which may be solid or liquid, intogaseous state; metal vapor traversing the space between the evaporationsource and the substrate at reduced gas pressure; condensation of thevapor upon arrival on the substrates. Accordingly, the theory of vacuumevaporation includes the thermodynamics of phase transitions from whichthe equilibrium vapor pressure of materials can be derived, as well asthe kinetic theory of gases which provides models of the atomisticprocesses. Further investigation of the sometimes complex eventsoccurring in the exchange of single molecules between a condensed phaseand its vapor led to the theory of evaporation, a specialized extensionof the kinetic theory. The distribution of deposits on surfacessurrounding a vapor source can be derived. The kinetic aspects ofcondensation processes are a topic in their own right relating tonucleation and growth phenomena. Vacuum deposition and its applicationshave benefited from various disciplines which have contributed towardsolutions of practical problems. These pertain to the design of suitablevapor sources and the development of special techniques for theevaporation of metals, alloys, compounds, and a variety of vacuumsystems with specialized process controls and automation.

The transition of solids or liquids into gaseous state may be treated asa macroscopic or as an atomistic phenomenon. The former approach isbased on thermodynamics and yields a quantitative understanding ofevaporation rates, interactions between evaporants and their containers,stability of compounds, and compositional changes encountered during theevaporation. The atomistic approach is derived from the kinetic theoryof gases and provides models which describe evaporation processes interms of properties of individual particles. The latter theory alsoapplies to the evacuation of vessels. Thermodynamic and kinetic theoriesare treated in various textbooks and often time research data will varyfrom text to text.

Coating properties depend on deposition procedures which become quitecomplex and must be monitorized closely from cycle to cycle to maintainconsistency. Every detail is important and details of procedures areusually proprietary. Relevant coating properties related to depositionof any metal consist of (1) coating structure, (2) internal stress and(3) adhesion. As any metallic coating grows from ultra-thin (10angstroms, 0.001 microns) to relatively thick, thin-films (up to 500,000angstroms, 50 microns) various coating zones result from the interactionof the basic processes that occur during deposition, coating fluxtransport, and surface and bulk diffusion. When a coating is applied toa substrate, stresses usually develop within the coating and at theinterface, consisting primarily of superimposed thermal and intrinsicstresses. The mechanisms of adhesion between metal coatings and organicpolymers (thermoformed substrates) are typical of those that aregenerally observed, classed as mechanical or interlocking, weak boundarylayer, chemical, and electrostatic. Film discoloration, substratewarpage, even total part destruction, can easily occur if the product orvaporization time and current is too high, the load is not large enough,or part placement is not optimal. Firing too long or at an excessivelyhigh current can burn or thermal warp the substrates.

When vacuum deposition is used to coat thin wall thermoformed shapes,care must be taken in the deposition process.

During the metal deposition cycle heat is generated and the distancefrom the deposition source to the thin-walled thermoformed piecesbecomes a critical factor. In an evacuated chamber, there is littleconduction or convection of heat but the radiant energy from theevaporant source can distort, warp, and even melt the polymer forms,especially in corner, or deep draws where the film is drawn to itsthinnest dimension. Thermal properties and wall thickness of thethermoformed film, heat output of the evaporant source, distance fromsource to substrate, duration of vaporization, and rotation speed of thesubstrate are all variables which need consideration.

The critical factors which need to be considered when vacuum metallizingthermoforms are:

-   -   1) Thermal properties and thickness of the wall of the        substrate.    -   2) Method and heat dissipation of evaporation.    -   3) Duration or time cycle of deposition.    -   4) Pressure within the chamber.    -   5) Type and amount of material being vaporized.    -   6) Speed or movement past the vaporization source.    -   7) Substrate distance from the evaporant source.

Transfer of heat within a solid, liquid, or gaseous media from onemedium to another can occur by conduction, convection, or radiation.When coating within a vacuum, conduction and convection becomeinsignificant and radiant heat transfer is the significant attribute tocontrol. Thermoformed shapes not corresponding to any definite or simplegeometry generally do not constitute cases of unidimensional heattransfer. The method of vaporization and time also affects the amount ofenergy released from the vaporization source.

The following table shows a comparison of the typical energies for thedifferent types of vacuum deposition sources. Type of DepositionElectrical Vapor State Thermal Energy Thermal Evaporation ElectricallyNeutral 0.1 to 0.3 eV Ion Beam High Negative Potential  10 to 40 eV(3000 to 4000 V) Sputtering High Negative  10 to 40 eV (Variable)

The radiant heat output from both ion beam and sputtering along with theextended cycle times needed to deposit relatively thick thin films makethese methods less commercially feasible than the thermal evaporationmethod. Thermoformed polymers have much lower thermal conductivities andthermal diffusities than injection molded parts and are more susceptibleto damage from the radiant heat and cycle times necessary for the othertwo methods of evaporation, namely sputtering and ion beam.

For practicable deposition rates, source materials should be heated to atemperature so that its vapor pressure is at least 1 Pa (10⁻² torr) orhigher. Table A represents these temperatures for several commonelements. TABLE A TABLE TEMPERATURE FOR VAPOR PRESSURE OF 1 Pa(10-2torr) Temperature Element ° C. ° F. Aluminum 1150 2100 Beryllium1245 2270 Cadmium 265 510 Carbon 2460 4460 Chromium 1400 2550 Copper1260 2290 Gold 1400 2550 Indium 945 1730 Lead 715 1320 Magnesium 440 825Manganese 940 1720 Molybdenum 2350 4260 Nickel 1530 2780 Platinum 20903790 Silicon 1470 2680 Silver 1630 2970 Tantalum 3060 5540 Tin 1250 2280Titanium 1740 3160 Tungsten 3230 5840 Zinc 345 650 Zirconium 2400 4350

Aluminum is the most common element used for vacuum deposition andbecause of its various properties, aluminum is used in as many as ninetypercent of the deposition applications. Although at a pressure of 1 Paits vapor temperature is 2100° F., it is normally vaporized at atemperature over 3000° F. (1649° C.). Actual results have provenaluminum vaporized most efficiently at vapor temperatures between 3300°F. to 3600° F. This is approximately 50 percent above the actual vaportemperatures (at 1 Pa). Other elements react relatively similarlyalthough they may have their own idiosyncratic characteristics. Whenvapor temperatures are very high and vapor source temperatures are veryhigh, as hot gaseous vapor flux travels from the source towards thesubstrate, its temperature rapidly dissipates with the distancetraveled. Surface measurements made on thermoforms twelve inches fromthe source indicate that the vapor flux temperature is reduced toapproximately 185° F. to 190° F. (85° to 90° C.) as it condenses on thepart surfaces. It is interesting to note that vapor source temperaturesare normally ten to twenty-five times greater than the heat deflectiontemperature of the substrate polymers being processed.

The relative ease of vacuum coating any part is related to its shape orconfiguration, its position relative to the vapor source (vapor flux)and its distance from the source. FIG. 8 shows that maximum coatingthickness is obtained at a centerline to the substrate. As the angle ofincidence increases, the thickness decreases rapidly dropping to lessthan fifty percent at a forty-five degree angle. At more than aforty-five degree angle, the coating density and adhesion are also verypoor.

Distance from the vapor source to the part to be coated is the mostimportant process variable and the easiest to control. Although thevapor source may be movable in some equipment designs, it is usuallyfixed. Substrate tooling and fixturing set-ups are normally convenientlyarranged to be adjustable to accommodate distance changes for differingsubstrates. It is well known in the art that when decorative coatings(less than 0.1 microns thick) are applied to injection molded parts(parts with wall thickness greater than 0.040 inches) a distance fromeight inches to twelve inches between vapor source and part should bemaintained to prevent warpage and burning. The thicker the part andhigher the thermal properties, the closer the part may be placed to thevapor source.

Our experiments with thin-walled thermoforms (0.006 to 0.020 inchesthick) and EMI functional coatings (over 1.0 microns thick) havedeveloped the results listed in the following table B. A minimumdistance of twelve inches should be maintained for even the highestthermal property films. TABLE B DISTANCE FROM VAPOR SOURCE TO SUBSTRATEIN INCHES FOR VARIOUS THERMOFORMED RESINS¹ HEAT DEFLECTION MINIMUM RESINTEMP. ° F. WALL DISTANCE FROM SYMBOL NAME (UNFILLED RESIN)² THICKNESSVAPOR SOURCE PI Polyimide 460-480 .006 in. 12 to 14 inches LCP LiquidCrystal 428-465 .006 in. 12 to 14 inches Polymer PEI Polyetherimide387-392 .010 in. 12 to 14 inches PS Polysulfone 345 .010 in. 12 to 14inches PC Polycarbonate 250-270 .010 in. 14 to 16 inches PBTPolybutylene 248-266 .010 in. 14 to 16 inches Terphthalate PPSPolyphenylene 221 .012 in. 16 to 18 inches Sulfide ABS Acrylonitrile-170-220 .012 in. 16 to 18 inches Butadiene- Styrene HIPS High-Impact170-205 .012 in. 16 to 18 inches Polystyrene PETG Glycol- 145-151 .015in. 18 to 20 inches Modified Polyester PVC Polyvinyl 130-150 .015 in. 18to 20 inches Chloride PP Polypropylene 120-140 .015 in. 18 to 20 inches¹Samples processed in 66 inch diameter vacuum metallizer with 30 kVApower source at fifty percent power, 20 tungsten wire vapor sources with1200 mg. of aluminum per filament, 30 second evaporant time, over 3000°F. flash temperature at a chamber pressure of 1 Pa (10-2 torr).²ASTM D 648, Heat deflection temperature under flexural load, 264 P.S.I.

Polyimide (PI) and liquid crystal polymer (LCP) represent the most heatresistant thermoforms coated. A six millimeter wall thicknessthermoformed shape of PI placed at a distance of twelve inches from thevaporization source was found to be the limiting combination of wallthickness and distance to evaporant source which could be achievedwithout heat distortion. It should also be noted that specialized orcustom thermoforming equipment is necessary to form shapes from thesematerials because they require higher forming temperatures with longerheating and cooling cycles.

The more commonly used thermoformed materials such as high-impactpolystyrene, polypropylene, ABS, polyvinyl chloride, and PETG have muchlower thermal properties. A minimum wall thickness of 0.012 to 0.015inches is required and working distances of from 14 inches to 18 inchesshould be maintained. In the cases of polypropylene and PVC atwall-thicknesses of 0.015 inch, it is also advisable to reduce the powersetting to the evaporant source by twenty-five percent and increase thetime cycle by twenty-five percent to prevent warpage (primarily due to“hot spots” in the vapor flux). FIG. 9 shows relative coating thicknessas a function of vapor source-to-part distance comparing injectionmolded parts to various thermoforms.

It should be understood that thin-walled thermoforms from polymericsheet of thicknesses from 0.006 inches to 0.100 inches are contemplatedby this invention to be metal coated and used for EMI shielding, as arethick-walled thermoforms having wall thickness in excess of 0.100inches.

Recycling

Two options for recycling metallized inserts are available. The first ischemical stripping and the second is simply regrinding and re-extrudingthe scrap for reuse.

Vacuum deposited aluminum is easily removed with solutions of potassiumand sodium hydroxide. These spent solutions containing aluminum can bediluted or neutralized with acid. Solutions with a pH under twelve whichcontain no heavy metals can be released to a sanitary sewer systemwithout any treatment. Other deposited metals require pretreatmentdepending on their concentrations.

A better alternative is to simply shred and regrind the metallizedthermoforms. This material can then be re-extruded into roll or sheetform (as would also be done with the trimmed metallized scrap). There-extruded material has thus been filled with metal. This material isused to form new inserts which would be deposited with metal on theirexterior surfaces again. In effect the material becomes more conductivethe more it is recycled. Recycling would be the recommended manner ofdisposing of inserts metallized with metals and alloys other thanaluminum.

EXAMPLE

Eight samples were tested for shielding effectiveness using the“modified MIL-STD-285” method. In this procedure, samples to be testedare mounted in a test opening in the wall of shield room. The tests arerun by radiating the test samples with a signal generator and antennainside the room, and measuring the levels outside the shield room with aspectrum analyzer and antenna. A baseline measurement was made throughthe open hole, without any samples in place. The difference in these twomeasurements—before and after the samples are installed in thehole—yields the “shielding effectiveness.”

Certain errors can be introduced in this method at lower frequencies.The “hole” itself provides shielding when the longest dimension is lessthan one half wavelength, but since values are subtracted from the“baseline”, the errors give more conservative lower levels of shieldingthan free space measurements. In this case, these errors only affectedthe 30 and 50 MHz measurements, making those measurements moreconservative by 15 dB. The remaining measurements, from 60 MHz to 3 GHz,are not affected. (NOTE: This is not considered significant, but isincluded for clarity.)

Tests were performed in three ranges, as follows:

Range I—30 MHz-200 MHz. Test equipment consisted of biconical antennas,signal generator/amplifier, and spectrum analyzer. As discussed earlier,some reduction is caused by the “hole” at frequencies below 60 MHz.

Range II—200 MHz-1 GHz. Test equipment consisted of log period antennas,signal generator, and spectrum analyzer. No reductions due to “hole”effects.

Range III—1 GHz-3GHz. Test equipment consisted of microwave hornantennas, microwave signal generator, and spectrum analyzer. Noreductions due to “hole” effects.

Electric field measurements were made on all samples at selectedfrequencies from 30 MHz to 3 GHz. This data is summarized in Table 2,and FIGS. 10-14.

Surface impedance measurements were made on all samples using a Keithlymicro-ohmmeter. Measurements were made approximately one inch apart atnine locations on each sample (top-middle-bottom by left-center-right).The nominal averages are summarized in Table 1. A prediction for planewave shielding is also included, based on the following formula:SE=20 log(Zw/4Zs)

-   -   SE is the shielding effectiveness in dB    -   Zw is the wave impedance (assumed as 377 ohms)    -   Zs is the surface impedance (ohms/square)

This formula is a “first approximation,” but the predicted data may beuseful in correlating the test data.

Finally, magnetic field measurements were made on two samples atselected frequencies from 10 KHz to 20 MHz. Since low frequency magneticfield shielding normally requires thick steel or mu-metals, littleshielding was anticipated. TABLE 1 SAMPLE DESCRIPTIONS No.Material/Finish Surface Impedance SE* #1 Silver acrylic paint  0.07ohms/square 62 #2 Silver vacuum deposition  0.04 ohms/square 67 Smooth -1 side, 1 coat #3 Aluminum vacuum deposition  0.15 ohms/square 55Smooth - 2 sides, 2 coats #4 Aluminum vacuum deposition  0.12ohms/square 58 Bead blast - 1 side, 1 coat #5 Aluminum vacuum deposition 0.03 ohms/square 70 Smooth - 1 side, 2 coats #6 Phosphor bronze(copper-tin)   0.5 ohms/square 45 Vacuum deposition Smooth - 1 side, 1coat #7 Aluminum vacuum deposition  0.16 ohms/square 55 Smooth - 2sides, 1 coat #8 Silver acrylic paint  0.05 ohms/square 65 — Aluminumfoil 0.0004 ohms/square 107 — Copper tape 0.0003 ohms/square 110*First order shielding approximation for plane waves. Does not accountfor different frequencies.

Test Results

The test data is summarized in Table 2, and is shown graphically inFIGS. 10-14. TABLE 2 TEST DATA SUMMARY Shielding Effectiveness, dBElectric Field Tests F (MHz) #1 #2 #3 #4 #5 #6 #7 #8 Room 30 53 63 53 4659 33 55 53 68 50 46 60 52 42 51 28 51 48 68 60 47 61 54 41 51 27 54 4882 70 46 60 54 41 51 27 54 47 85 80 49 61 58 44 53 29 57 50 78 100 48 6158 42 53 29 58 48 76 130 57 67 64 50 60 35 65 57 81 170 53 63 63 46 5734 66 55 77 200 59 69 66 50 64 37 69 55 80 200 63 73 84 58 71 44 81 6586 450 60 66 69 56 66 44 68 64 74 600 58 71 82* 53 66 40 80* 60 80 80060 69 76 59 66 45 79* 59 79 1000 66 78* 71 58 65 45 73 63 78 1000 68* 6767 65 68* 53 67 67 69 1500 89* 83 81 67 69 53 82 77 89 2000 67 80 91* 6069 46 93* 70 91 2500 68 80 87* 63 77 50 86* 73 86 3000 61 68 73 58 66 4875 67 82 Magnetic Field Tests Frequency #3 #4 Room  10 KHz 0 0 16 150KHz 0 0 25  1 MHz 4 0 32  20 MHz 24 20 32*Measurement limited by shield limits.

FIG. 10 compares the silver acrylic paint with the silver vacuumdeposition coating. The silver vacuum coating performed approximately 10dB better than the silver paint. Both samples #1 and #8 used silverpaint, and show consistent test results. Sample #2 is the silver vacuummaterial.

FIG. 11 compares the aluminum vacuum coating, one side versus two sideswith one coat of material. The double sided sample performsapproximately 10 to 20 dB better. This is believed due to the secondreflection that occurs at the second surface.

FIG. 12 compares the aluminum coating, one side versus two sides withtwo coats of material. These are quite close in performance. It wasnoted that the single side sample had a very low surface impedance (0.03ohms/square) while the double sided sample had higher surface impedances(0.15 ohms/square). Had the surface impedances been the same, a largerdifference would be expected. Nevertheless, one can conclude from thisthat two thin coats perform as well as one thick coat.

FIG. 13 compares aluminum, silver, and phosphor bronze. All threesamples here were single side, single coat. Not surprisingly, the silverdoes the best.

For solid shields, copper would normally be expected to perform betterthan aluminum. In this case, however, the copper did not appear to bedeposited in a uniform manner. Also, the surface impedance was muchhigher than the silver or aluminum.

One can conclude from this graph that while the silver does the best,the aluminum performs quite well. Unless the copper deposition problemscan be overcome, aluminum is preferred.

FIG. 14 shows the brief magnetic field tests done on two samples.Although very little shielding effectiveness was expected, it appearsthe materials start to provide some magnetic field shielding above 1MHz. This is consistent with other thin non-ferrous materials.

CONCLUSIONS

From the test data, the following conclusions were reached:

All of the samples performed quite well in the electric field testing.With the exception of the phosphor bronze sample, all other samplesprovided 40-60 dB of shielding. This should be more than adequate formost present commercial computer designs.

The silver vacuum sample performed better than the silver acrylic paint,providing over 60 dB of shielding. This would be very useful for highperformance requirements, such as high speed computers or radiotransmitters/receivers.

The double sided aluminum samples also performed quite well, providingover 50 dB of shielding. This is believed due to the double reflectionsprovided by metallizing two surfaces. Metallizing both sides would berecommended when additional shielding is desired.

The copper (phosphor bronze) sample did not perform as well as thealuminum. This may be due to non-uniform deposition on the surface.Given the overall good performance from the aluminum and silver, theremay not be a need for a copper deposited option.

The lower the surface impedance, the better the shielding. It alsoappears that the surface impedance correlates quite well with predictedand measured shielding effectiveness. While not a final indicator,surface impedance (ohms/square) is a useful parameter to predict finalshielding effectiveness.

Very little low frequency magnetic field shielding was provided by thesesamples, although some magnetic field shielding is provided above 20MHz. This is not a surprise, since normally this requires thick steelfor shielding.

1. An EMI/RFI shielding device comprising: a shaped polymer substratecomprised of metallized polymer substrate, wherein the shaped polymersubstrate is substantially conductive; and a conductive material on atleast one surface of the shaped polymer substrate.
 2. The EMI/RFIshielding device of claim 1 wherein the recycled metallized polymersubstrate comprises a reground and re-extruded metallized thermoform. 3.The EMI/RFI shielding device of claim 1 wherein the conductive materialhas a thickness between 1.0 micron and 50.0 microns.
 4. The EMI/RFIshielding device of claim 1 wherein the conductive material comprisesaluminum.
 5. The EMI/RFI shielding device of claim 1 wherein theconductive material comprises a substantially uniform thickness over atleast one surface of the shaped polymer substrate.
 6. The EMI/RFIshielding device of claim 1 wherein the shaped polymer substrate has athickness between 0.006 inches to 0.100 inches.
 7. An EMI/RFI shieldcomprising: a thermoformed thin-walled shape formed of a recycledmetallized polymeric material, wherein the thermoformed thin-walledshape comprises an inner surface, an outer surface and edges; and aconductive material deposited on at least one of the inner surface andouter surface, wherein the conductive coating comprises a substantiallyeven thickness between 1 micron to 50 microns.
 8. The EMI/RFI shield ofclaim 7 wherein the conductive material comprises vacuum depositedaluminum.
 9. The EMI/RFI shield of claim 7 wherein the recycledmetallized polymeric material comprises a reground and re-extruded metallayer and a polymeric material.
 10. The EMI/RFI shield of claim 7wherein the thermoformed thin-walled shape has a thickness between 0.006inches to 0.100 inches.
 11. The EMI/RFI shielding device of claim 1further comprising grinding and re-extruding a metal material along withthe polymer substrate.
 12. The EMI/RFI shielding device of claim 1wherein the conductive material comprises copper.
 13. The EMI/RFIshielding device of claim 1 wherein the conductive material comprisesnickel.
 14. The EMI/RFI shield of claim 7 wherein the conductivematerial comprises vacuum deposited copper.
 15. The EMI/RFI shield ofclaim 7 wherein the conductive material comprises vacuum depositednickel.